Mysql – Does thesql “order by” guarantees to give the same order for the same query every time if the sort key is equal

MySQL

I'm planning to write a query that sorts the result by a value s and then paginate the result. Let's say I have ten items that match the query and all the items have the same s. In the first query, I sort the value by s and get the first five items. Then in the next query, I sort the value by s and get the sixth to tenth items. Is it possible that the items that appear on my first query will appear again in my second query, and that some items will not show up in either query?

Best Answer

When the field being sorted on is not unique, then the order of the results are non-deterministic, meaning there is no guarantee on their exact ordering. Specifically in your example case where the first 10 results all share the same exact value for the s field could yield indeterminate results for the first time you run the query vs the second time. It is possible you see the same result in both cases or not at all as you guessed. You would need to add another field to the ORDER BY clause that uniqifies the sort in order to always guarantee deterministic results.

For a little proof of case and further reading, here is a DBA.StackExchange question and answer where the user was experiencing unpredictable ordering of results because the field they were sorting on wasn't unique. The comments on that answer briefly mention that being the cause.

Related Solutions

Mysql – User-defined sort order for a query

How about using a prepared statement to do it? Something like the below:

SELECT @orderBy := orderby, @ascDesc := ascdesc FROM user WHERE UserId = @id;
PREPARE cmd 
FROM 'SELECT * FROM user WHERE user.UserID = ? ORDER BY ? ?';
EXECUTE cmd USING @id, @orderBy, @ascDesc;
DEALLOCATE PREPARE cmd;

EDIT

Apparently it is not possible to use parameters in the way that I have above (see comment below) in which case you should do the following:

SELECT @orderBy := 'Name', @ascDesc := 'ASC' FROM user WHERE Id = @id;
SET @s := 
CONCAT('SELECT * FROM user WHERE user.UserID = ? ORDER BY ',@orderBy,' ',@ascDesc);
PREPARE cmd FROM @s
EXECUTE cmd USING @id;
DEALLOCATE PREPARE cmd;

I hope this helps you.

MySQL – Does Join Occur Before LIMIT?

The only way around it is to refactor the query so as to alter the order of the EXPLAIN plan.

EXAMPLE : On May 16, 2011, I once answered a very rough question on StackOverflow

Fetching a Single Row from Join Table

Question : I had to join tables together and return only the 40 most recent results.
My Answer : I proved that you can do a JOIN on tables with large rows and get results fast.
How I did it : I had applied principles of refactoring queries I learned from a Youtube Video.

Basically, you have to construct the query so that JOIN clauses are done last. Consequently, this implies that you must get the LIMIT executed beforehand.

Best Answer

Related Solutions

Mysql – User-defined sort order for a query

MySQL – Does Join Occur Before LIMIT?

Fetching a Single Row from Join Table

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